The present invention relates to systems and apparatus for drilling boreholes in the earth for the ultimate recovery of useful natural resources, such as oil and gas. More particularly, the invention relates to apparatus and methods for reaming a borehole and for stabilizing a drilling assembly. Still more particularly, the invention relates to apparatus and methods that include reaming and back reaming a borehole to have a diameter that is larger than the inside diameter of the casing string or open hole that is above the borehole.
In the drilling of oil and gas wells, it is frequently necessary or desirable to “ream” a borehole that has been previously created by a drill bit or other cutting tool so as to remove formation projections that may have survived the first pass of the drilling assembly and to thereby provide a relatively smooth and more uniform borehole wall surface. In certain applications, a reamer is placed behind the drill bit on the drilling assembly so as to ream the hole immediately after the bit has formed the borehole. It is sometimes preferred that such a reaming step be performed as the bit is being withdrawn from the borehole, such process being referred to as “backreaming.” An alternative to backreaming is to withdraw the bit and then run into the hole a drill string having a reamer on the end. This, of course, requires an extra trip of the drill string and thus is costly and undesirable in most cases.
Ensuring a relatively smooth borehole well is particularly important to ease the installation of well casing. In the drilling process, concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. In supporting the additional casing strings within the previously run strings, the annular space around the newly installed casing string is limited. Further, as successively smaller diameter casings are suspended within the well, the flow area within the casing for the production of oil and gas is reduced. To increase the annular area for the cementing operation and to increase the production flow area, it has become common to drill a larger diameter new borehole below the terminal end of the previously installed and cemented casing string. Enlarging the borehole beneath the previously installed casing string permits the installation of new casing that is larger than that which could otherwise have been installed in the smaller borehole. By drilling the new borehole with a diameter that is larger than the inside diameter of the existing cased borehole, a larger annular area is provided for the cementing operation. Further, the subsequently suspended new casing may itself have a larger inner diameter than otherwise possible so as to provide a larger flow area for the production of oil and gas.
Various methods and apparatus have been devised for passing a drilling assembly through the existing cased borehole, yet permitting the assembly to then drill a new borehole that is larger in diameter than the inside diameter of the upper, existing cased borehole. One such method is to use under reamers, which are tools that are collapsed to pass through the smaller diameter of the cased borehole and thereafter expanded to ream the new borehole and provide a larger diameter for the installation of new casing. Many conventional under reamers employ concentric bodies and pivotable arms that, in certain instances, have tended to break during operation. When this occurs, the broken components must be “fished” from the hole before drilling can continue, thus greatly increasing the time and cost required to drill the borehole. Another such method is to employ a winged reamer disposed above a conventional bit. Still another method for drilling a larger diameter borehole is to employ a drilling assembly that includes a bi-center bit.
The bi-center bit is a combination eccentric reamer section and pilot bit. The pilot bit is disposed on the lowermost end of the drilling assembly with the reamer section disposed above the pilot bit. The pilot bit drills a pilot borehole on center in the desired trajectory of the well path, and then the eccentric reamer section follows the pilot bit, reaming the pilot borehole to the desired diameter for the new borehole. The diameter of the pilot bit is made as large as possible to provide stability, but it is not made so large as to prevent the combination of pilot bit and winged reamer from passing through the cased borehole. Certain conventional such bi-center bits drill a borehole that is approximately 15% larger than the diameter of the existing cased borehole. However, since the reamer section is eccentric, the reamer section tends to cause the bit axis angle to slightly shift during its rotation, thus pointing the bit in different directions, and therefore to deviate from the desired trajectory of drilling the well path. Also, the bi-center bit also tends to be pushed away from the center of the borehole because of the resultant force of the radial forces acting on the reamer blade (caused by weight on bit and by the circumferential forces caused by and acting on the cutters on the pilot bit) Also, the direction and magnitude of these radial forces change as drilling parameters such as RPM, weight on bit, hole inclination, and formation type change, which influences directional tendencies of the bit. In certain formations, these lateral forces can cause the pilot bit to drill its portion of hole oversize, and thus the reamer section of the bi-center bit to drill an undersized hole.
It is well understood that to control the direction of drill path, stabilizers are provided on the drill string. By appropriately positioning a stabilizer of a particular design, the trajectory of the drill path can be better controlled. In certain drilling circumstances, it is desirable to place a stabilizer adjacent to the bi-center bit. However, space limitations in the casing, through which all components of the drilling assembly must pass has, in the past, prevented the placement of a “near-bit” stabilizer adjacent to a bi-center bit.
U.S. Pat. No. 6,213,226, (the entire disclosure of which is hereby incorporated by reference into this application), describes an eccentric, adjustable blade stabilizer that may be placed close to a bi-center bit in order to stabilize the bit and to effect the drilling of a larger bore hole in the desired trajectory beneath a section of a previously-cased borehole. The apparatus described therein includes extendable blades that, once below the previously-cased borehole and into the newly formed borehole, expand to the full gage diameter of the new borehole to provide enhanced stability for the bi-center bit and to align the pilot bit with the axis of the existing borehole. Also incorporated by reference into this application is U.S. Pat. No. 6,227,312.
Conventional bi-center bits, however, cannot effectively be used to “back ream” the newly formed borehole because of a lack of adequate stabilization. More specifically, as the drilling assembly having the bi-center bit is withdrawn, the pilot bit does not provide the stabilization needed to cause the winged blade to ream properly. Instead, the forces imposed by the formation material on the wing of the bi-center bit pushes the drilling assembly off center once the pilot bit has been withdrawn from the pilot hole and enters the region of the newly formed borehole having the larger diameter. Thus, the reamer of the bi-center bit is not sufficiently stabilized by the pilot bit to permit effective back reaming. Accordingly, the new section of the borehole has to be drilled correctly and entirely in a single pass, or else a second trip of the drill string would be required to conduct a reaming procedure.
In certain formations, it is also desirable or necessary to drill an enlarged borehole beneath a previously-drilled and uncased (open) borehole. This is because certain formations are sensitive to the increased fluid pressures that result from smaller hole diameters. Such higher pressures or fluctuations in pressures may cause sloughing off of formation material into the borehole. Accordingly, to lessen the likelihood of such an occurrence, it is known to drill a larger diameter borehole at locations beneath open holes having a smaller diameter so as to reduce the equivalent circulating density (“ECD”) of the drilling fluid. Thus, it would thus be desirable to develop a drilling assembly that can be employed to drill an enlarged borehole beneath a cased section or beneath a previously drilled open hole where the assembly can also be used to back ream the newly formed and enlarged hole.
A particular use of a bi-centered bit is in drilling out the casing shoe. A casing shoe is placed on the lowermost end of a casing string and is used to guide the casing string into the wellbore since there may be partial obstructions in the wellbore, such as ledges, for example. The typical casing shoe includes a generally cylindrical steel casing having an internally threaded upper box portion for connection to a complementary pin portion of a casing string. The lower end of the shoe includes a central portion made of drillable material (such as cement, aluminum, plastics or the like) and a generally rounded nose projecting frontwards, beyond the forward or lowermost end of the casing.
Upon installing and cementing a casing in a newly drilled borehole, the casing shoe attached to the lower end of the casing also becomes cemented into the borehole. Thus, to drill a new borehole below the cased borehole, it is necessary that the shoe and remaining cement first be drilled out. It was once standard practice to drill through the casing shoe using a standard drill bit, then to remove the bit from the hole, install a bi-center bit on the drill string and run it into the cased borehole, and then to drill the enlarged hole beneath the installed casing. However, that practice required an extra trip of the drill string and thus was time consuming, costly and undesirable. More recently, specialized bits have been developed for drilling through the casing shoe, and then continuing to drill to form an enlarged hole beneath the cased borehole. This allowed the new borehole to be created without requiring an additional trip of the drill string to attach a bi-center bit. One such bit said to be designed for drilling out the casing shoe and continuing on to drill the enlarged borehole beneath the installed casing is disclosed in U.S. Pat. No. 6,340,064.
In general, the specialized bits for drilling through the casing shoe are a form of a bi-center bit, the bit having a first pilot bit and a set of offset cutters axially disposed from the pilot bit and extending radially beyond the diameter of the pilot bit. However, without a near bit stabilizer, the specialized bit for drilling the casing shoe could not provide back reaming as the bit is removed from the borehole due to the formation pushing the drilling assembly off center, as previously discussed.
To drill the casing shoe, the drill string is rotated as drilling fluid is pumped down through the drill string and out the face of the bit, the fluid returning to the surface along the annulus formed between the drill string and the casing wall. For use after the bi-center bit has passed through the casing and begun to cut the enlarged borehole, it would be desirable to include in the drilling assembly a near-bit, eccentric, adjustable blade stabilizer, such as that disclosed in U.S. Pat. No. 6,213,226. The stabilizer disclosed therein, however, includes means for extending the blades upon increasing the pressure of the drilling fluid passing through the drill string. In other words, the blades are retained in a contracted position by spring force until a predetermined drilling fluid pressure causes them to extend.
When drilling out the casing shoe using a bi-center bit, it is important, therefore, that the stabilizer blades not be extended prematurely. However, when drilling through the cement or other material of the casing shoe, high fluid pressure may be required as compared to that used merely to pass the drilling assembly to the bottom of the existing casing. This increase in fluid pressure could cause the extendable stabilizer blades of a stabilizer such as that disclosed in U.S. Pat. No. 6,213,226 to extend prematurely, detrimentally effecting the ability of the bit to drill out the casing shoe. Alternatively, premature blade extension while the shoe is being drilled may damage the stabilizer blades, rendering them ineffective or less effective in guiding the bit along the intended drilling path after the casing shoe has been drilled out. Accordingly, where a near bit, eccentric, adjustable blade stabilizer is employed, it would be desirable to provide a means to ensure that the blades do not extend prematurely, and that they remain in their completely retracted position until a predetermined control is sent from the surface to the drilling assembly.